1. Field of the Invention
The invention relates to electronics and, more particularly, to image intensifiers designed to investigate high-speed processes in single-frame recording and photochronograph modes of operation.
The image intensifier of the invention can find use in diagnostic electrooptical apparatus applicable to experimental work in the field of physics of lasers and laser plasma, thereby permitting investigation of phenomena relating to controlled laser thermonuclear synthesis, laser photobiosynthesis, laser spectroscopy, and laser location.
2. Description of the Prior Art
Known in the art are image intensifiers comprising an evacuated tube envelope which accommodates a serial arrangement of a photocathode, a focusing electrode in the form of a truncated cone member, an anode diaphragm, an electronic gate in the form of two pairs of deflecting plates separated by a gating diaphragm, two deflection systems disposed at right angles to each other, and a fluorescent screen.
The prior art image intensifiers provide for practically inertialess control of the electron image, which is attained by virtue of an electric field produced by the control plates of the electronic gate and the deflection system.
The scene to be investigated is displayed on the screen at a higher time resolution with the result that separate phases thereof follow one after another with relatively small time intervals of the order of 10.sup.-11 s.
There exist, however, rapid and extremely rapid processes requiring picosecond time resolution. In this case, adequate image intensifiers must resolve optical signals that follow one after another with time intervals of 10.sup.-12 s and less.
In the photochronograph mode, the scattering of the transit times of the electrons required for their passage from the photocathode to a plane where they are deflected is mainly responsible for a limitation of time resolution.
On the other hand, the above-mentioned scattering depends on the scattering of the initial velocities of the electrons and a lesser magnitude of the latter provides for a better time resolution.
A reduction of the scattering of the transit times of the electrons, with other parameters of the image intensifier tube held constant, is attained by increasing the intensity of the electric field near the photocathode (cf. British Pat. No. 1,329,977, cl. H1J, 31/50).
The image intensifier as disclosed in the last-mentioned reference comprises an evacuated tube envelope which accommodates a serial arrangement of a photocathode, an accelerating electrode having a fine-mesh grid disposed near the photocathode, a focusing electrode in the form of a truncated cone member, an anode diaphragm, an electronic gate in the form of two pairs of deflecting plates separated by a gating diaphragm, an image scanning system, and a fluorescent screen.
The fine-mesh grid, having at least 300 meshes per mm.sup.2 and being spaced from the photocathode by a distance of 1 to 3 mm, is maintained at a positive potential, relative to the latter, of the order of one kilovolt, thereby providing for an increase in the intensity of the electronic field near the photocathode.
However, the employment of the grid at a higher potential results in a two-fold increase of the magnification of the image intensifier. As a result, the screen luminance is decreased by a factor of 4 in the single-frame recording mode while the resolution referred to the screen is decreased by a factor of 2 in the photochronograpgh mode.
In addition, the focusing electrode, in this case, must be held at a low potential to provide for the creation of the image in the screen plane, which tends to slow down the movement of the electrons in the space between the grid and the anode diaphragm. This slow movement of the electrons results in a greater scattering of their transit times.
Another factor to be taken into consideration is the dependence of the magnification of the image intensifier on the potential at which the accelerating electrode is maintained.
Therefore, a decrease in the scattering of the transit times of the electrons, attained due to the fact that the intensity of the electric field near the photocathode is increased, is not realized to the fullest extent and does not reach a magnitude necessary for the subpicosecond range of resolution.
As an example, consider the difference .DELTA.t between the transit times t.sub.o and t for two electrons, respectively, that move in the known image intensifier in the space between the grid and the plane of the electron beam deflection. ##EQU1## E is the intensity of the electric field near the photocathode; .DELTA.t is the difference between the transit times for two electrons;
m is the mass of electron; PA1 e is the charge of the electrode; PA1 U.sub.o =mV.sub.oz.sup.2 /2e PA1 V.sub.oz is the normal component of the initial velocity of electron escaping from the photocathode surface; PA1 U.sub.(z) is the potential distributed along axis z; PA1 t.sub.o the transit time of the electron possessing a velocity V.sub.oz =0; PA1 t is the transit time of the electron possessing a velocity of V.sub.oz .noteq.0; PA1 z.sub.1 is the grid coordinate; PA1 z.sub.2 is the coordinate of the plane of the electron beam deflection. PA1 L=(7.1 to 8.6)d PA1 S.sub.4 =(0.10 to 0.14)d PA1 S.sub.2 =(0.08 to 0.19)d PA1 l.sub.1 =(1.1 to 1.45)d PA1 l.sub.2 =(0.57 to 0.77)d
The factor .eta. represents the number of times by which the difference .DELTA.t is increased as compared with the case when the deflection plane coincides with the grid plane.
The factor .eta. equals 1.75 in the case of the electrooptical system of the known image intensifier. This means that the scattering of the transit times of the electrons is increased by 75 percent. That value of the factor .eta. is indicative of the number of times by which the time resolution is decreased because of a low potential at the focusing electrode, as compared to the calculated value of the resolution corresponding to the given intensity of the electric field near the photocathode.
The deflecting plates of the image scanning system and the electronic gate are incorporated, in the known image intensifier, in the control circuit in the form of terminating capacitive loads whose passband is limited by their resonance frequency.
In the known image intensifier, it is impossible, therefore, to obtain the speeds of image scanning which would be equal to one or two velocities of light and which would provide for adequate recording of phenomena within the picosecond and subpicosecond ranges of time resolution.
Where the scanning speed increases due to an increase in the pulse amplitude, with the result that the latter exceeds the amplitude necessary for scanning the image over the working portion of the screen, the electron beam, during deflection, impinges on the walls of the evacuated envelope of the image intensifier tube. As a result, the electrons subjected to elastic reflection from the walls cause a decrease in the image contrast and the time resolution is also decreased.